Method of improving the corrosion resistance of titanium metals



United States Patent assignor to Imperial Chemical Industries Limited, L'ondon, England, a corporation of Great Britain No Drawing. Filed July 17, 1958, Ser. No. 749,052 Claims priority, application Great Britain July 26, 1957 4 Claims. (Cl. 204147) This invention relates to a method of improving the corrosion resistance of titanium and titanium-base alloys.

It is well known that in chemical plant and other industrial applications the use of the morecommon structural metals is severely limited by the poor corrosion resistance of such metals to the generality of electrolytes. In the past therefore it has been the practice to protect such metals with a surface coating of a more corrosion resistant or more noble substance In the case of such coatings it has been emphasized that the coverage of the metal must be complete since the presence of pores or other discontinuities in the surface layer gives rise to the formation of galvanic couples and corrosion of th underlying metal results.

For some years now, titanium has taken a prominent place as a structural metal because of its outstanding corrosion resistance to many electrolytes. There remain however some important electrolytes in which the corrosion rates of titanium and its alloys prohibit their use for handling such media, e.g. as storage tanks and other equipment.

According to the present invention a method of reducing the rate of corrosion of a structure of titanium or a titanium-base alloy of comparable anodic polarization properties to titanium in contact with a liquid medium Patented Aug. 27, 1963 2 stages of rolling powderedmetal to sheet, precautions being taken to ensure even distribution over the surface.

When carbon is the cathodic material, it may take the form of discrete particles embedded in the surface, each particle being capable of protecting a surrounding area many times its size. The protection given appears to be of a permanent nature and any scratches or damage of a similar character to the surface of the metal become sealed over by a protective film. This is opposite to the effect normally produced by coating a metal with a more noble or cathodic metal.

It will of course be appreciated that the more noble material must be employed in a form of suitable electrochemical character. Since the passage of current in most cases will require the evolution of hydrogen at the more noble material, the surface of this material must have a comparatively low overvoltage. Platinum therefore may well be used in the formof a platinum black. Smooth platinum coated with platinized platinum is especially suitable.

The amount of galvanic current produced as abovementioned and therefore'the resulting potential assumed by the titanium will depend inter alia on the electrolyte and the temperature. Cases may'arise therefore where the potential produced in this way is not sufficiently positive to give the required degree of protection to the titanium. Whether the potential produced is sufilcient for the purpose in hand may readily be reduced 'by reference to the potential/corrosion rate graph which can be .plotted by experiments conducted with titanium specimens for the particular electrolyte and set of operatingconditions. This graph is believed to take generally the same form for many electrolytes and it is found that after an initial increase of corrosion rate corrosive thereto comprises electrically-connecting the tective against corrosion by the liquid medium. One

example of a titanium-base alloy which can be protected according to the invention is an alloy of titanium and Zirconium containing 5% by weight of the latter.

' According to one embodiment of the invention the current passing to the titanium to reduce its rate of corrosion may be produced by the galvanic couple formed between the titanium and more noble material when contact is made therebetween. In such a case the contact between the two parts may be direct, i.e. by the attachment of the more noble material to the titanium surface, or indirect, i.e. by means of a wire or other lead external to the electrolyte.

Electrical contact between the titanium and the more noble element may with advantage be effected by in corporating the element in the surface of the metal and for this purpose a number of methods are available. 'For metals such as platinum, functioning as a cathode, attachment may be achieved by plating'or spot-welding or vacuum deposition or spraying techniques. In the case of non-metallic materials such as carbon, deposition by decomposition of carbon monoxide orcarbon dioxide vapours, or coating with oil and subsequently carbonising may be carried out. Bonding the metal to pieces of graphite is also suitable. A method which can be used for both metals and non-metals is to introduce the cathodic material as a small quantity of powder during the last with potential, there is a more or less rapid drop towards a condition in which the titanium is substantially completely passive. --In cases therefore in which the poto give the requisite protection will vary with the cir- 45 cumstanoes. Many cases, a potential of 1.0 volt positive to the standard calomel electrode will give virtually complete protection and, for most electrolytes, it will rarely be necessary to raise the potential to more than about 3 volts. The power costs of operating the present invention are small in relation to the saving of plant and materials that is effected thereby.

The invention has special application to the improvement of the corrosion resistance of titanium in nonoxidizing acids.

The invention is illustrated further in the following examples.

EXAMPLE 1 A titanium strip 5 ems. long x 1 cm. wide is electrically connected to a graphite cathode and immersed in 40% w./w. sulphuric acid at 60 C., the graphite being partly immersed therein. The electrical connection is by way of an external wire spot-welded to the titanium at one end and secured 2Jl'."ll'.S other end as a push fit in a small hole drilled in the graphite. The corrosion of the titanium expressed in inches penetration per year is 0.0035 as compared with the value of 1.12. inches for an unprotected control sample immersed in the solution.

EXAMPLE 2 The procedure of the previous example is repeated employing in place of the graphite a smooth platinum electrode coated with platinum black. The corrosion rate of the titanium as compared with that of an uncoupled control sample is reduced by a factor of approximately one-third.

EXAMPLE 3 A large tank constructed from titanium sheet and maintained at a positive potential of 3.0 volts with respect to a graphite cathode is protected from corrosion by 40% W./w. sulphuric acid contained within the tank at a temperature of 60 C. The arrangement employs a graphite cathode partly immersed in the sulphuric acid in the tank, the electrical cell so formed being connected with a continuously trickle-charged battery. The power consumption is approximately 3 watts per thousand square feet of immersed titanium surface. Comparative tests carried out for a period of the order of 700 hours indicate that the corrosion rate of the titanium is 7x 10 inches per year as compared with 1 inch per year for an unprotected specimen of titanium.

The accompanying table shows the degree of protection afforded to titanium in other electrolytes.

Corrosion Rates of Titanium in Various Media, With and Without Applied Positive Potential We claim:

1. A method of reducing the rate of corrosion of a structure of a metal selected from the group consisting of titanium and titanium base alloys having anodic polarization properties comparable to those of titanium, said structure being in contact with a liquid medium corrosive thereto and comprising a member of the group consisting of sulphuric, phosphoric, hydrochloric, oxalic and formic acids, said method comprising immersing a conductive material selected from the group consisting of carbon and metals of the platinum group in said liquid at a point remote from said metal structure, and electrically-connecting said structure and said material so that direct current passes between the structure and said material and through the liquid medium to increase 4 the potential of the structure to a value at which a protective film is maintained on the surface of said structure by anodic polarization, said potential being not substantially in excess of the minimum value required to maintain said film on the surface of said structure.

2. A method of reducing the rate of corrosion of a structure of a metal selected from the group consisting of titanium and titanium base alloys having anodic polarization properties comparable to those of titanium, the structure being in contact with a liquid medium corrosive thereto and comprising a member of the group consisting of sulfuric, phosphoric, hydrochloric, oxalic and formic acids, said method comprising placing said structure in direct contact with a conductive material selected from the group consisting of carbon and metals of the platinum group, said conductive material also being in contact with said liquid medium, whereby the galvanic couple formed between said structure and said conductive material causes direct current to pass between them and through the medium to increase the potential of the structure to the galvanic cell potential at which a protective film is maintained on the surface of the structure in contact with the liquid medium by anodic polarization, said potential being not substantially in excess of the minimum value required to maintain said film on the surface of said structure.

3. A method of reducing corrosion as set forth in claim 1 including increasing the potential difference between said structure and said non-ferrous material sufficiently to maintain said protective film by means of a source of direct current electricity electrically connected between said structure and said non-ferrous material, the positive connection being to said structure.

4. A method of reducing corrosion as set forth in claim 3 in which the potential of said structure is from 1 to 5 volts positive to the standard calomel electrode.

References Cited in the file of this patent UNITED STATES PATENTS 1,077,920 Stevens Nov. 4, 1913 1,477,099 Baum Dec. 11, 1923 1,787,672 Davenport Ian. 6, 1931 2,719,797 Rosenblatt et a1. Oct. 4, 1955 2,795,541 Muller June 11, 1957 2,871,425 Burnham Jan. 27, 1959 FOREIGN PATENTS 774,598 Great Britain May 15, 1957 OTHER REFERENCES Cotton: Platinum Metals Review, vol. 2, No. 2, April 1958, pages -57. 

1. A METHOD OF REDUCING THE RATE OF CORROSION OF A STRUCTURE OF A METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND TITANIUM BASE ALLOYS HAVING ANODIC POLARIZATIN PROPERTIES COMPARABLE TO THOSE OF TITANIUM, SAID STRUCTURE BEING IN CONTACT WITH A LIQUD MEDIUM CORROSIVE THERETO AND COMPRISING A MEMBER OF THE GROUP CONSISTING OF SULPHURIC, PHOSPHORIC, HYDROCHLORIC, OXALIC AND FORMIC ACIDS, SAID METHOD COMPRISING IMMERSING A CONDUCTIVE MATERIAL SELECTED FROM THE GROUP CONSISTING OF CARBON AND METALS OF THE PLATINUM GROUP IN SAID LIQUID AT A POINT REMOTE FROM SAID METAL STRUCTURE, AND ELECTRICALLY-CONNECTING SAID STRUCTURE AND SAID MATERIAL SO THAT DIRECT CURRENT PASSES BETWEEN THE STRUCTURE AND SAID MATERIAL AND THROUGH THE LIQUID MEDIUM TO INCREASE THE POTENTIAL OF THE STRUCTURE TO A VALUE AT WHICH A PROTECTIVE FILM IS MAINTAINED ON THE SURFACE OF SAID STRUCTURE BY ANODIC POLARIZATION, SAID POTENTIAL BEING NOT SUBSTANTIALLY IN EXCESS OF THE MINIMUM VALUE REQUIRED TO MAINTAIN SAID FILM ON THE SURFACE OF SAID STRUCTURE. 